But that is the point, Ethniu gave Mab everything she had with the Eye, and Mab withstood it. Against an immortal with half a brain to reason, the Eye isn't deadly, yes, Mab took a severe shot, but it gave the others a window in which to take her down.
She took a single shot, and survived. Just because she could take one shot, doesn't mean the eye isn't deadly. She probably wouldn't have survived a second, as she was made clear to Harry that he needed to act quickly. If it weren't deadly to her, she wouldn't have been worried because she could have taken more blasts.
The Eye is clearly deadly to her. She is just powerful enough to take one good blast from it. A second would have finished her. She was pretty clear that it could kill her when Harry said she was Immortal, and she replied that she wasn't eternal when discussing the Eye and it's power.